Question 1

What is the difference between odds ratio and relative risk?

Accepted Answer

Odds ratio compares odds (probability of event / probability of no event), while relative risk compares probabilities directly. In case-control studies, only the odds ratio can be calculated. In cohort studies, both can be used. When the outcome is rare (< 10%), OR approximates RR well.

Question 2

What does it mean when the confidence interval includes 1?

Accepted Answer

If the 95% CI includes 1, the odds ratio is not statistically significant at the 5% level. This means the observed association could be due to chance. An OR of 1 means no difference between groups, so including 1 in the CI means you cannot rule out no association.

Question 3

How do I handle zero cells in the contingency table?

Accepted Answer

Zero cells cause problems (division by zero or infinite OR). The Haldane correction adds 0.5 to each cell: use (a+0.5)(d+0.5) / (b+0.5)(c+0.5). This provides a finite estimate and valid confidence interval. Some researchers prefer exact methods instead.

	Outcome+	Outcome-
Exposed	a	b
Unexposed	c	d

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2x2 Table

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